checked class applied on change event

relative to : https://github.com/arthurgouveia/prettyCheckable/issues/28

I think it makes more sense to set the "checked" class regarding the input change event rather than the click event. Then to programmatically set a prettyCheckbox state, we just have to call :
```
$input_wrapper.find(":checkbox").prop("checked", isChecked).change();
```

This kinda redirects the click event to the input tag, so the "disabled" behaviour should be handled natively now.

Jasmine tests passed 5/5 (very useful BTW)
This is my first pull request, so I hope I didn't make any mistakes.